Symmetrical carbocyanine dyes



2,739,149 SYMMETRICAL CARBOCYANINE DYES- Earl J. VanLare, Rochester, N;Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporationof New Jersey No Drawing. Serial No. 339,459

Application February 27, 1953,

This invention" relates to symmetrical carbo'cyanine dyes containing abenzirnidazole'nucleus; More particularly; thisinvention relates tosymmetricalcarbo'cy'anine dyes containing a benzimidazole 'nucleushavin-g substituted-thereon at least one chlorine'atom.

Symmetrical imiclazolocarbocyanine dyes have been previously prepared.See Ogata-Proceedings of-the Imperial Academy (Tokyo), vo1ume'9-(19 33),page 602.

I have -now found a new class of symmetrical carbocyanine dyescontaining a benzimidazole nueleus having substituted thereon at leastonechlorine atom; The new dyes of my invention have been found to beparticularly useful in altering the sensitivity of-photographic silverhalide emulsions, particularly in. the blue-green region of thespectrum; Thesensitizing bands of the newdyes of my invention areunusually sharp, hence they can be used expeditiously in sensitizingphotographic silver halide emulsions where little or no-sensitivity inthe red region of the spectrum is desired.

The new symmetrical carboeyanine dyes of my invention can advantageouslybe represented by the following general formula:

wherein R is an alkyl group, such --as methyl, ethyl, .npropyl,,n-butyl, allyl (vinylmethyl), carbethoxymethyl, etc., or an aryl group,such as phenyl,-o-, .mor 'p-tolyl, etc. (e: g. a mononuclear aromaticgroupof the=benzene series), R1 represents an alkyl group, suchas-methyl, ethyl, n propyl, n-butyl, allyl (vinylmethyl), carbethoxyamethyl,-etc., X represents an acid radical, such as chloride, bromide,iodide, p-toluenesulfonate, benzenesulfonate, ethylsulfate,methylsulfate, perchlorate; thiocyanate, etc., and n representsapositive integer of from 1 to- 2. The dyes of Formula I wherein R andRI each representsflan alkyl group containing from 1 to 4carbonatomshave been found to be particularly useful.

According to my invention, the dyes represented by Formula I above canadvantageously be'prepared by condensing-together a quaternary-saltselected from those represented by the following general formula:'

wherein R, R1, X and n each have the values given above,

with diethoxymethyl acetateorchloral alcoholate. In-

of ether,

2,739,149 Patented Mar. 20, 1956 stead) of using diethoxymethyl acetate,other dialkoxymethyl acetates, e. g. dimethoxymethylacetate; etc., canbe used. The" condensationof the quaternary salts of Formula II withchloral alcoholate are advantageously carried'out in the presence'of aninert diluent, such as the 'alcoholfrorn which the alcoholate isobtained. For example, inert diluents such as ethanol, n-propanoh. 11-

butanol, etc., can be employed. The condensations are advantageouslycarried out in the presence of a strong base, such as an alkali metalalcoholate, e. g. sodium ethylate, potassium ethylate, etc. Heataccelerates the condensations, although temperatures varying from roomtemperature to the reflux temperature of the reactionmixture can beemployed.

The condensation ofthe quaternary salts" of Formula II with adialkoxymethyl acetate is advantageously carried out in the presence ofa basic condensing agent, such as pyridine, quinoline, triethylamine,N,N-diethylaniline, N-methylpiperidine, etc. Heat accelerates thecondensation, although temperatures varying; from room temperature tothe refiuxtemperature of the reaction mixture can be employed.

The following examples will serve to illustratemore tully'the mannerwhereby I prepare the new carbocyanine dyes of my invention.

Example- 1 .5 ,5 -dich l0r0-1,1- Q33-tetraethylbenzimidazolocarbocyanineiodide Method A.-5-chloro-1,3-diethyl-2-methylbenzimidazolium iodide(3.5 g., 2 mols.), and chloral alcoholate (1 g., 1 mol.) were added toasolution of sodium (0.6 g., 3 mols.) in 60 ml. of ethanol and themixture refluxed for 30 minutes. The hot solution was filtered and thesolid washed well with water. A crude yield of 1.8 g. 62% of dye wasobtained. After recrystallization from ethyl alcohol, the dye wasobtained in -a 29% yield as orange crystals, M. P.271-272 C.'dec.

Method B.--5'-chlore 1,3 diethyl-Z-methylbenzimidazc lium iodide (3.5g., 2 mols.), diethoxyrnethyl acetate (1.6

g., l mol.+%"excess') and 25 ml. of pyridine were The solution waschilled and some unchanged, quaternary salt filtered ott. The crude dyewas precipitated from the filtrate by addition After filtering andwashing well with water, the crude dye was recrystallized from ethylalcohol. The purified dye was obtained in a 4% yield as red plates.

Example 2.5,5',6,6'-tetrachl0r0-1 ,1',3,3'-tetrzrethylbenzimidazolocarbocyanine iodide M ethodA.--5,6-dichloro--1,3-diethyh2-methylbenzirnidaz'olium iodide'(7.7 g., 2mols.) and chloral alcoholate (2 gt, 1 mol.) were added to a solution ofsodium (1.4 g:, 3 'mols.) in l20 ml. ofethyl alcohol-and themixture'refinned for- SO-mi'riutest The reactionmixture was chilled,filtered, and the solid washed well-with=water and acetone;

After recrystallization from methyl alcohol, the pure dye was obtainedin a 32% yield as lustrous crystals, M. P. 275278 C. dec.

Method B.-5,6-dichloro-1,3-diethyl-2-methylbenzimidazolium iodide (7.7g., 2 mols.), diethoxymethyl acetate (3.2 g., l mol.+l% excess) and 70ml. of pyridine were refluxed together for 2 /2 hours. The solution wascooled and the crude dye filtered off and washed well with water andacetone. After recrystallization from methyl alcohol, the pure dye wasobtained in an 18% yield.

Example 3.5,5'-dichloro-1,1,3,3-tetrameth ylbenzimidazolocarbocyanineiodide 5 chloro l,2,3-trimethylbenzimidazolium iodide (5.4 g., 2 mols.)and choral alcoholate (1.6 g., 1 mol.) were added to a solution ofsodium (2.3 g., 6 mols.) in 100 ml. of ethyl alcohol, and the mixturerefluxed 45 minutes. The crude dye was filtered from the hot reactionmixture and washed Well with water. After recrystallization frompyridine, the dye was obtained in a 4% yield as dark reddish crystals,M. P. 288-289 C. dec.

Example 4 .1 ,1 ',3,3'-tetrallyl-5 ,5 -dichlorobenzimidazolocarbocyanineiodide 1,3 diallyl 5 chloro 2 methylbenzimidazolium iodide (7.5 g., 2mols.), diethoxymethyl acetate (3.2 g., l mol.+100% excess) and 30 ml.of pyridine were refluxed together for 4 hours. After cooling, the dyewas precipitated with water, filtered oil and washed Well with water.After recrystallization from methyl alcohol, the pure dye was obtainedin a 20% yield as dark red crystals with a green reflex, M. P. 234236"C. dec.

Example 5 .5 ,5 -diehloro-3,3 '-diethyl-1 ,1-dipherzylbenzimidozoloearbocyanine iodide 5 chloro 3 ethyl 2 methyl lphenyl benzimidazolium p-toluene sulfonate (4.4 g., 2 mols.),diethoxymethyl acetate (1.6 g., 1 mol.+l00% excess) and 10 ml. ofpyridine were refluxed together for 3 hours. After cooling, the reactionmixture was treated with 100 ml. of ether. The supernatant liquid wasdecanted and the oil was washed again with ether and then dissolved in asmall amount of ethyl alcohol. The dye was converted to the iodide bythe addition of a hot solution of 3 g. of sodium iodide in water. Aftercooling, the dye was filtered oil and washed with water. Afterrecrystallization from ethyl alcohol, the pure dye was obtained in ayield as dark red crystals with a green reflex, M. P. 280281 C. dec.

4 AnaL-Calculated for C3sH29Cl2IN4: C, 58.3; H, 4.3. Found: C, 58.5; H,4.7.

Example 6 .5 ,5 ',6,6 '-tetrachlor0-3,3 '-diethyl-1 ,1diphenylbenzimidazolocarbocyanine bromide 5,6 dichloro 3 ethyl 2 methyll phenylbenzimidazolium p-toluenesulfonate (4.8 g., 2 mols.),diethoxymethyl acetate (1.6 g., 1 mol.+l00% excess) and 15 ml. ofpyridine were refluxed together for 2 hours. After cooling, the reactionmixture was treated with ml. of ether. The supernatant liquid wasdecanted oil and the oil washed again with ether. The oil was dissolvedin a small amount of ethyl alcohol and converted to the bromide by theaddition of a hot solution of 3 g. of sodium bromide in water. Aftercooling, the dye was filtered off and washed well with water. Afterrecrystallization from ethyl alcohol, the dye was obtained in a 23%yield as dark red crystals with a green reflex, M. P. 272-273 C. dec.

The following examples will serve to illustrate the method of preparingcertain intermediates employed in the above examples.

Example 7.-5,6-diehloro-Z-methyl-I-phenyl benzimidazole CnHi I G1- o-onCIQN/ 2,4,S-trichloronitrobenzene g., 1 mol.) and aniline (130 g.) wereheated on a steam bath for 65 hours. Three hundred (300) ml. of waterand 100 ml. of 40% sodium hydroxide were added and the mixture steamdistilled to remove excess aniline. When no more oil steam distilled,the residue was cooled, and the supernatant water decanted, as much aspossible. The residue was dissolved in 1500 ml. of glacial acetic acid,to which 250 ml. of acetic anhydride were added. The solution was heatedto refluxing and zinc (275 g., 6 mols.) was added in small quantitieswith stirring. A vigorous reaction occurred with each addition. Afterall of the zinc was added, the mixture was heated and stirred for anadditional hour. The zinc acetate was filtered off and washed withacetic acid. The filtrate was concentrated to a small volume and pouredinto 1 liter of water. It was made strongly alkaline with sodiumhydroxide. The suspension was extracted with 4 liter of benzene, thebenzene solution concentrated to 700 ml. and cooled. The solid wasfiltered off and washed with benzene. The benzene solution wasconcentrated to dryness and the residue distilled. B. P. l60-2l0 C./0.5mm. A 17% yield of waxy solid was obtained. A small sample was purifiedby recrystallization from ethyl alcohol, M. P. l07-l08 C.

Example 8.-5,6-dichloro-3-ethyl-2-methyl-J- phenylbenzimidazoliump-toluenesulfonate CuHs C OSOaCiHt rag-7393: 149

;6-'dichloro=2 methyl-[1-phenylbenzimidazole (32"'g., 1 mol.) and ethylp-toluenesulfonate (32 g., 1 mol.+40% excess) were heated on a:steambath for 3 hours. The melt was cooled and stirred with 300 ml. ofacetone. After cooling, the solid was filtered oil and washed withacetone. A 44%. yield of white crystals was obtained, M. P. 229-232 SIdec.

Example 9.-5-chl0r0-2-methyl-1-phenylbenzimidazole (30352,5-dichloronitrobenzene (233 g., 1 mol.) and aniline (226 g;, 2'mols.)were heated to 150 C. for 2 hours. Finely pulverized sodium carbonate(64 g., 1 mol.) was added and the wholegent-ly reftuxed for 4 hours. Thereaction mixture-was cooled and the excess aniline steam distilled out.The residue was cooled and the super natant-water decanted oil. Theresidue was suspended iii 690*ml." oi concentrated hydrochloric acid and600 ml: of'water. Mossy-tin (346 g., 3 /2 mols.) was added to-thehot;stirred suspension. A good bit of "heat was developed 'and the mixtureboiled. After all the tin was added, the mixture was heated and stirred4 hours on a steam bath." After cooling, the mixture was made stronglyalkaline withsodium hydroxide, and-extracted with ether. The. ether wasremoved and the residue mixed with 400 ml. of acetic anhydride. Thetemperature slowly rose and finally, the. acetic acid boiled. The aceticacid and excess acetic anhydride were slowly distilled off. The residue'was poured into water and made alkaline with sodium hydroxide. It wasextracted with ether and distilled. B. P. ISO-460 C./0.5 mm. Thedistillate was crystallized. from ligroin (B; P-. 90120) to give 342.7'grto 21% of product, M. P. 115117 C.

Awake-Calculated for C14H11C1N2: C, 69.3; H, 4.6. Found: C, 69.7; H,4.9.

Example'l0. 5 chloro 3 ethyl 2 methyl 'y phenylbenzimidazoliump-toluenesulfonate GaHs ' o-cm (BROSOaC-IHKD) 5-chloro-2-methyll-phenylbenzimidazole (12.1 g., 1 mol.) and ethyl p-toluenesulfonate g.,1 mol.) were heated together for 3 hours at 140 C. It was then refluxedwith acetone for minutes. The cake was ground in a mortar with acetone,filtered and washedwith acetone. A yield of 20.7 g. 93% of whitecrystals was obtained.

2,5,-dichloronitrobenzene (107 g., 1 mol.) was added slowly withstirring to 70% ethylamine (200 ml., 4 mols.). After all of the materialwas added, the mixture was heated in a water bath at 40-50 C using acoil condenser fonIZh'ours; The m'aterialwas chilled, filtered, andwashed-with water. It was=stirred with a small amount of methyl alcoholandfiltered. A sample was crystallized frommethyl alcohol and found tohavea- M. P of 89 -9d C. A 96% yield was obtained Example I2.-5-chlor0-1-ethyl-2-methylbenzimidazole Mossytin (363 g., 3 mols.) was addedslowly: to a hot (-100 C.) stirred suspension of 4-ch1oro-N-ethyl-2-nitroaniline (221v g., 1 mol.) in 920 ml. of concentrated hydrochloricacid and 1200 ml. of water. A vigorousreaction occurred and after theaddition, the mixture wastefluxed with stirring for 2 hours. The mixturewas cooled and made strongly alkaline with 40% sodium hydroxide. Thecooled suspensionwas filtered, and the solid and liquid extractedseparately with benzene. The benzene extractswere combined, thebenzeneremoved on a steam bath .under reduced pressure, and the residueof 4-chloro- N-ethyl-o-phenylene diamine used crude.

One-half of the crude o-phenylene diarnine (theory of 94 g.) and 200 ml.of acetic anhydride were slowly mixed.

A good bit of heat was developed and the mixture boiled. Theacetic-acidand excess-acetic anhydride were slowly distilled oil. The residue waspoured into 2 liters of water and .made alkalinewith sodium hydroxide.The

product was extracted with ether/the ether solution dried with magnesiumsulfate, and then the ether removed on asteambath', The residuewas-distilled. A90% yield was obtained, B. P. 165 470 C./4 mm.Crystallization from ligroin (B. P. 90-120C.) yielded a product M. P.84-86 C.

Example 13. 5 chloro -,1,3 diethyl 2 methylbenzimi'dazolitzm iodideS-chloro-l-ethyl-2-methylbenzimidazole (20 g., 1 mol.) and ethyl iodide(23 .4' g., 1 mol.+25% excess) were heated together on a steam bath for2 hours. The solid cake was crushed in a mortar and washed with ether.It was filtered oil and washed with acetone. A 78% yield of colorlesscrystals was obtained, M. P. 250-251 C. dec. Recrystallization frommethyl alcohol did not change the M. P.

Example 14.-4,5-dichlor0-N-ethyZ-Q-nitroaniline I 01 -NH-C 2H2,4,S-trichloronitrobenzene (155 g., 1 mol.) was added with shaking to300 ml. of 70% ethylamine (4.4 mols.). After standingtor /2 hour at roomtemperature under a coil condenser, the suspension became quite warm andthe reaction was cooled in ice'water. After cooling, it was allowed tocome to room temperature and stand for /4 hour, when it became necessaryto cool again; It was then stood at room temperature overnight. Thesolid was filtered oil and washed with water. A nearly-quantitativeyield oforange crystals was'obtain'ed, M1" P. -103 C.

7 Example 15. 5,6 dichloro 1 ethyl 2 methylbenzimidazole C-CH;

Mossy tin (288 g., 3 /2 mols.) was added slowly (with stirring) to a hot(90100 C.) suspension of 4,5-dichloro- N-ethyl-2-nitroaniline (161 g., 1mol.) in 390 ml. concentrated hydrochloric acid and 750 ml. water. Afterall the tin was added, the mixture was refluxed for 2 hours. Thereaction mixture was cooled and made strongly alkaline with 40% sodiumhydroxide. The cooled suspension was filtered, and the solid and liquidextracted separately with benzene. The benzene extracts were combinedand the benzene removed under reduced pressure. To the residue of5,6-dichloro-N-ethyl-o-phenylene diamine were added 200 ml. of aceticanhydride portion-wise. The solution was allowed to stand overnight andthen the acetic acid and excess acetic anhydride distilled off. Theresidue was poured into 2 liters of water and made strongly alkalinewith sodium hydroxide. The product was extracted with ether, the ethersolution dried over magnesium sulfate, and the ether removed on a steambath. The residue was distilled, B. P. l75-200 C./2 min. The distillatewas melted and poured into ligroin. After filtering, the benzimidazolewas obtained in a 55 yield as colorless crystals, M. P. 80-85" C.

Example 16. 5,6 dichloro 1,3 diethyl 2 methylbenzimidazolium iodide CzHsI 5,6-dicl1loro-l-ethyl-2-methylbenzimidazole (22.9 g., 1 mol.) andethyl iodide (23.4 g., 1 mol. ,50% excess) were refluxed together for 4hours. The solid cake was ground in a mortar with acetone, filtered andwashed well with acetone. A 78% yield of colorless crystals wasobtained.

Example 1 7 -chIor0-2-methylbenzimidazole Mossy tin (400 g., 3 mols.+10%excess) was added slowly with stirring to the hot (90-100 C.) suspensionof 4-chloro-2-nitroaniline (172.5 g., 1 mol.) in 830 ml. concentratedhydrochloric acid and 1100 ml. water. After all the tin had been added,the mixture was refluxed 3 hours, then cooled and made strongly alkalinewith 40% sodium hydroxide. The o-phenylene diamine was extracted withether, the ether removed on a steam bath and the residue refluxed 4hours with 500 ml. of acetic anhydride. The reaction mixture was pouredinto ice and water and made alkaline with sodium hydroxide. The solidwas filtered oil, and crystallized from methyl alcohol. The solid wasplaced in a fiask and heated very hot whereupon acetic acid distilledover. After /2 hour, the residue was crystallized from ethyl alcohol. An18% yield of colorless crystals was obtained, M. P. 2l5218 C.

Anal.-Calculated for CsHrClNa: C, 57.7; H, 4.2. Found: C, 57.4; H, 3.8.

8 Example 18. 5 chloro 1,2,3 trimethylbe/zzimidazolium iodide ExampleI9.N-allyl-4-chl0r0-2-nitr0aniline -NHCHaCH=CHg 2,5-dichloronitrobenzene(96 g., 1 mol.) was added in small portions to allylamine (57 g., 2mols.). The reaction mixture was heated overnight on a steam bath, andthen cooled. The solid filtered off and washed with water. Aftercrystallization from ethyl alcohol, a 76% yield of orange crystals wasobtained, M. P. 5860 C.

Example 20.-1-allyl-5-chloro-Z-methylbenzimidazole C HzC H=C Hi o-oH, 01N/ Zinc dust (148 g., 6 mols.) was added slowly to a hot stirredsolution of N-allyl-4-chloro-2-nitroaniline (80.7 g., 1 mol.) in 800 ml.of acetic acid and ml. acetic anhydride. After all the zinc was added,the mixture was refluxed with stirring for 2 hours. The mixture wasfiltered hot and the solid washed with hot acetic acid. The filtrate wasconcentrated to a small volume and poured into 1 liter of Water. Thesolution was made strongly alkaline with sodium hydroxide, and extractedwith ether. The ether was removed on a steam bath and the residuedistilled. A 55% yield of waxy solid was obtained, B. P. 195 C./12 mm. Asample was recrystallized from ligroin (90l20 C.), M. P. 93-94 C.

Example 21. 1,3 diallyl 5 chloro 2 methylbem zimidazolium iodide1-allyl-5-chloro-2-methylbenzimidazole (41.3 g., 1 mol.) and allyliodide (42 g., 1 mol.|25% excess) were heated together on a steam bathfor 4 hours. The product was ground in a mortar with acetone, filteredand washed with acetone. A 75% yield of colorless crystals was obtained.

In addition to the diethoxymethyl acetate illustrated above otherdialkoxymethyl acetates which can be used in practicing my invention canadvantageously be represented by the following general formula:

wherein R2 represents an alkyl group. Such esters have been previouslydescribed in Dent and Brooker U. S. Patent 2,537,880, issued January 9,1951.

All of the dyes of my invention are particularly useful in manufacturingphotographic, silver halide emulsions, serving to alter the sensitivitythereof. Sensitization by means of my new dyes is, of course, directedprimarily to the ordinarily employed, gelatino-silver-halide, developing-out emulsions. The dyes are advantageously incorporated in thewashed, finished emulsion and should, of course, be uniformlydistributed throughout the emulsion. In the preparation of photographicemulsions containing my new dyes, it is only necessary to disperse thedyes in the emulsions. The methods of incorporating dyes in emulsion aresimple and well known to those skilled in the art of emulsion making. Itis convenient to add the dyes from solutions in appropriate solvents.The solvent must, of course, be compatible with the emulsion andsubstantially free from any deleterious effect on the lightsensitivematerials. Methanol has proven satisfactory as a solvent for themajority of my new dyes.

The concentration of my new dyes in the emulsion can vary widely, i. e.from about to about 100 mgs. per liter of flowable emulsion. Theconcentration of the dye will vary according to the type oflight-sensitive material in the emulsion and according to the effectsdesired. The suitable and most economical concentration for any givenemulsion will be apparent to those skilled in the art upon making theordinary tests and observations customarily used in the art of emulsionmaking.

To prepare a gelatino-silver-halide emulsion sensitized with one of mynew dyes, the following procedure is satisfactory: A quantity of the dyeis dissolved in methyl alcohol or other suitable solvent and a volume ofthis solution (which may be diluted with water) containing from 5 to 100mgs. of dye is slowly added to about 1000 cc. of agelatino-silver-halide emulsion, with stirring. Stirring is continueduntil the dye is uniformly distributed throughout the emulsion. Withmost of my new dyes, 10 to mgs. of dye per liter of emulsion suffices toproduce the maximum sensitizing eifect with the ordinary gelatinosilverbromide (including bromiodide) emulsions. With fine-grain emulsions,which include most of the ordinarily employed gelatino-silver-chlorideemulsions, somewhat larger concentrations of dye may be necessary tosecure the optimum sensitizing effect.

The above statements are only illustrative and are not to be understoodas limiting my invention in any sense, as it will be apparent that mynew dyes can be incorporated by other methods in many of thephotographic silver halide emulsions customarily employed in the art.For instance, the dyes can be incorporated by bathing a plate or filmupon which an emulsion has been coated, in the solution ofthe dye, in anappropriate solvent. Bathing methods, however, are not to be preferredordinarily.

Photographic silver halide emulsions which can advantageously besensitized by means of the new dyes of my invention comprise thecustomarily employed gelatinosilver-chloride,gelatino-silver-chlorobromide, gelatinosilver-bromide, andgelatino-silver-bromiodide developing-out emulsions.

Photographic silver halide emulsions, such as those listed above,containing the sensitizing dyes of my invention can also contain suchaddenda as chemical sensitizers, e. g. sulfur sensitizers (e. g. allylthiocarbamide, thiourea, allylisothiocyanate, cystine, etc.), variousgold compounds (e. g. potassium chloroaurate, auric trichloride, etc.)(see U. S. Patent 2,540,085; 2,597,856 and 2,597,915), various palladiumcompounds, such as palladium chloride (U. 8. 2,540,086), potassiumchloropalladate (U. S. 2,598,079), etc., or mixtures of suchsensitizers; antifoggants, such as ammonium chloroplatinate (U. S.2,566,245), ammonium chloroplatinite (U. S. 2,566,263), benzotriazole,nitrobenzimidazole, 5-nitroindazole, benzidine, mercaptans, etc. (seeMeesThe Theory of the Photographic Process, Macmillan Pub., page 460),or mixtures thereof; hardeners, such as formaldehyde (U. S. 1,763,533),chrome alum (U. S.

1,763,533), glyoxal (U. S. 1,870,354), dibromacrolein (Br. 406,750),etc.; color couplers, such as those described in U. S. Patent 2,423,730,Spence and Carroll U. S. application 771,380, filed August 29, 1947, nowU. S. Patent 2,640,776, issued June 2, 1953, etc.; or mixtures of suchaddenda. Dispersing agents for color couplers, such as those set forthin U. S. Patents 2,322,027 and 2,304,940, can also be employed in theabove-described emulsions.

What I claim as my invention and desire secured by Letters Patent of theUnited States is:

1. A symmetrical carbocyanine dye selected from the group consisting of5,5-dichloro-l,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide;5,5,6,6'-tetrachloro- 1,1',3,3'-tetraethylbenzimidazolocarbocyanineiodide; 5,5- dichloro l,l',3,3 tetramethylbenzimidazolocarbocyanineiodide; l,l,3,3'-tetrallyl-5,5-dichlorobenzimidazolocarbocyanine iodide;and 5,5'-dichloro-3,3-diethyl- 1,l'-diphenylbenzimidazolocarbocyanineiodide,

2. The symmetrical carbocyanine dye having the following formula:

3. The symmetrical carbocyanine dye having the following formula:

aHs 1 1:

5. The symmetrical carbocyanine dye having the following formula:

CH2OH=CH| I AH:CH=C

6. The symmetrical carbocyanine dye having the following formula:

References Cited in the file of this patent UNITED STATES PATENTSKumetal July 25, 1944 Dent Ian. 9, 1951

1. A SYNMETRICAL CARBOCYANINE DYE SELECTED FROM THE GROUP CONSISTING OF5,5''-DICHLORO-1,1'',3,3''-TETRETHYLBENZIMIDAZOLOCARBOCYANINE IODIDE;5,5'',6,6''-TETRACHLORO1,1'',3,3''-TETRAETHYLBENZIMIDAZOLOCARBOCYANINEIODIDE; 5,5''DICHLORI - 1,1'',3,3'' -TETRAMETHYLBENZIMIDAZOLOCARBOCY5,5''ANINE IDIDE;1,1'',3,3''-TETRALLYL-5,5''-DICHLOROBENZIMIDAZOLOCARBOCYANINE IODIDE;AND 5,5''-DICHLORO-3,3''-DIETHYL1,1''-DIPHENYLBENZIMIDAZOLOCARBOCYANINEIODIDE.